1. Field of the Invention
The present invention is directed to an analog-to-digital converter circuit, and in particular to an analog-to-digital converter circuit for digitization a high-frequency analog signal which is amplitude-modulated by a modulation signal.
2. Description of the Prior Art
An analog-to-digital converter circuit is disclosed in U.S. Pat. No. 4,851,842 which has an analog input to which an analog high-frequency signal can be supplied. The circuit includes an analog-to-digital converter unit which is connected to the analog input via an amplifier with a controllable gain, the analog-to-digital converter having a digital output connected to a controllable divider. The divider is controlled by a control signal which digitally divides the digital output of the converter by an amount which is a reciprocal of the gain of the amplifier unit. This control signal is supplied by a quantization unit, which is also supplied with the analog input signal, and which selects a control signal, which in turn selects the gain of the amplifier (and the divisor of the divider) dependent on the amplitude of the analog input signal.
High-frequency signals in the megahertz range, having high dynamics of up to 96 dB occur, for example, as echo signals in magnetic resonance systems (MR systems). The signal processing in the reception channel of such MR systems is therefore complicated. The dynamics of the reception signal are dependent on various parameters such as, for example, the weight of a patient, the slice position wherein a tomogram is to be produced, the type of pulse sequence which is employed, the type of reception antenna, etc. A digitization of the reception signal in conventional MR systems ensues only following an analog demodulation. In order to provide the desired dynamics in the digital output signal, this requires that high-resolution analog-to-digital converters of, for example, 16 bits and having a sampling rate of 500 kHz, be used. In order to optimally drive the demodulator and the analog-to-digital converter, a receiver adjustment is also undertaken before each measurement, which results in a presetting of the level of the reception signal. This presetting ensues in test measurements. A digitization of the reception signal as early as possible in the receiver chain would be desirable, preferably before a demodulation. If this were accomplished, a reception signal could then be digitally further-processed, at least beginning with an intermediate frequency level. Moreover, if the digitization could ensue with high dynamics, the aforementioned receiver adjustment would be superfluous, and the overall time which a patient must spend in the examination apparatus would be shortened.
The analog-to-digital circuit disclosed in the aforementioned U.S. Pat. No. 4,851,842 is for the purpose of improving the digitization of small signals in the audio range. As described in the that patent, the accuracy of the analog-to-digital conversion is poorest in the lower significant bits of the digital output signal, however, for small signals the total digital output signal may not have many places above the lower significant bits. This results in a relatively high signal-to-noise ratio in conventional analog-to-digital converters. The circuit disclosed in U.S. Pat. No. 4,851,842 is intended to prevent a deterioration of the signal-to-noise ratio in the digitization of small signals, and the range of dynamics of the overall digital-to-analog conversion is thus enhanced. This is effected by the use of a controllable amplifier which precedes the actual digital-to-digital analog unit, and by a controllable divider following the analog-to-digital converter unit. The amplifier and the divider respectively amplify and divide by factors which are reciprocal. The control signal, which sets the gain of the amplifier and the divisor of the divider, is generated dependent on the envelope of the analog input signal which is to be digitized. The known circuit disclosed in U.S. Pat. No. 4,851,842, however, is not suitable for analog-to-digital conversion of signals having frequencies in the megahertz range, as arise in diagnostic magnetic resonance technology.